Polarimeters are measuring devices which measure polarization-optical properties of samples. In particular, with a polarimeter the optical rotation of an optically active sample is measured.
The optical polarization rotation of a sample is dependent on the wavelength of the light which is radiating through the sample, which is also denoted as measuring light in the following. In this document, the term polarization rotation in short is also denoted as (optical) rotation. In order to make polarization-optical measurements comparable, the optical rotation has to be measured at a standardized wavelength. Even when the optical polarization rotation of a sample is measured correctly at the current wavelength of the polarimeter, a deviation from the desired standardized wavelength leads to a measuring error for the optical rotation which is caused by the sample. This measuring error is determined on the one hand by an error of the wavelength of the measuring light and on the other hand by the dependency of the optical rotation of the sample from the wavelength.
EP 0 536 985 A1 discloses a method for calibrating a polarimeter which, in a calibrated state, shall operate with an unknown wavelength, at a standardized wavelength. A sample shall be measured with the unknown wavelength and the parameters of the polarimeter shall be adjusted such that the measurement with the unknown wavelength results in measuring values which correspond to a measurement with the standardized wavelength.
US 2013 169964 A1 discloses a method by which, in a polarimeter, the current wavelength of a measuring light at the point in time of a polarization-optical measurement can be ascertained. For this purpose, two measurements are performed: in a first measurement, the optical rotation of the sample to be measured is determined, in the second measurement, in addition to the sample, a polarization rotator is pivoted in the beam path and thus the combined optical rotation of the sample and the polarization rotator is measured. From the difference between the polarization rotations which are obtained with both measurements, the optical rotation of the polarization rotator is obtained, measured at the current wavelength of the polarimeter. From the known wavelength dependency of the optical rotation of the polarization rotator, the current wavelength of the polarimeter can then be determined.
There may be a need to increase the accuracy for the determination of polarization-optical properties of a sample and in particular of a polarization rotation which is caused by the sample.